In the design and operation of modern high capacity vapor generating units, the recirculation of combustion gases is commonly employed as a means for altering the heat absorption pattern within the vapor generator to effect substantially constant outlet steam temperature conditions over a wide load range. The gas to be recirculated is generally withdrawn from a relatively cool region of the vapor generator (usually downstream from the economizer) and is reintroduced into the boiler by means of a suitable gas recirculation system including a fan and associated duct work.
Where coal or other high ash fuel is burned in the generator, the gaseous combustion products leaving the generator contain significant quantities of particulate matter which, if carried over into the gas recirculation system (or for that matter, into any other subsequent flue gas flow system), may precipitate serious erosion problems and, in addition, pollution problems as well.
As a consequence, it has been the industry practice to force the flue gas stream to undergo a drastic change in direction. Due to the combined influences of gravity and the entrained particles' own inertia, the particles tend to be thrown off by centrifugal force into a suitably positioned ash hopper.
The problem with current designs is that the recirculation ducts are usually positioned at the back end of the recirculating apparatus and are thus in close proximity to the generator burners. So situated, the ducts may interfere with the burner piping and with the withdrawal of the burner and lighter. As a result, replacement and maintenance of the burners and their ancillary equipment is often time consuming and difficult.
Clearly, an apparatus that permits expeditious flue gas recirculation while simultaneously overcoming the enumerated difficulty is desirable.